Black-robed Islamic extremists armed with AK-47 automatic rifles invaded Gao in wooden boats Sunday to launch a surprise attack on the most populous city in northern Mali, two weeks after French and Malian troops ousted the jihadists.

Gunfire echoed for hours across the city of mud-walled buildings. The combat started at about 2 p.m. in downtown Gao and the fighting was continuing as night fell. Later the sound of gunfire was replaced by the clattering of French military helicopters overhead.

The attack in Gao shows the Islamic fighters, many of them well-armed and with combat experience, are determined and daring and it foreshadows a protracted campaign by France and other nations to restore government control in this vast Saharan nation in northwest Africa.

The Islamic radicals fought against the Malian army throughout the afternoon and were seen roaming the narrow streets blanketed in sand and on rooftops in the centre of Gao, which had a population of 90,000 before the conflict caused thousands to flee.

Families hid in their homes. One family handed plastic cups of water through the locked iron gate to others hiding on their patio. Piles of onions lay unattended where market women fled when the Islamists arrived. There were no signs of civilian casualties.

The fighting appeared to centre near the police headquarters, where Malian soldiers with rocket propelled grenades traded fire with the combatants believed to be from the Movement for Oneness and Jihad in West Africa, or MUJAO. The only sound in the city was gunfire and the bleating of goats. Soldiers were positioned at every corner in the neighbourhood.

Ever since French forces took Gao on Jan. 26, the Islamic rebels had clashed with security forces on the city's outskirts. This was the first time they succeeded in entering the strategic city.

The Islamic fighters used pirogues, large wooden dugouts with motors, and other boats to cross the Niger River and penetrate Gao, according to French Gen. Bernard Barrera, who cited Malian officials.

Besides Gao, French and Malian forces have also retaken the fabled city of Timbuktu and other northern towns, pushing the Islamic extremists back into the desert, where they pose a constant threat to Malian and allied forces. But the Islamic fighters made strategic retreats and are dug into desert hideouts, from where they are expected to continue challenging the control of the cities by French, Malian and allied forces. Several African nations have contributed troops to battle the extremists, who imposed their harsh version of Islamic Shariah law when they controlled the northern cities.